Air intake duct of saddle-ridden vehicle

ABSTRACT

An air intake duct, of a saddle-riding type vehicle, in which a flow velocity of an intake air is less likely to be reduced to enable prevention of reduction of an efficiency of a supercharger, is provided. The air intake duct of a motorcycle serves to supply incoming wind as the intake air to the supercharger for a combustion engine. The air intake duct has a passage area that is gradually reduced from an air intake port at a front portion toward a downstream portion connected to the supercharger at a rear portion.

CROSS REFERENCE TO THE RELATED APPLICATION

This application is a continuation application, under 35 U.S.C §111(a)of international application No. PCT/JP2013/068913, filed Jul. 10, 2013,which claims priority to Japanese patent application No. 2012-155462,filed Jul. 11, 2012, the entire disclosure of which is hereinincorporated by reference as a part of this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an air intake duct for supplyingincoming wind as an intake air to a combustion engine of a saddle-ridingtype vehicle.

2. Description of Related Art

An engine, mounted to a saddle-riding type vehicle such as a motorcyclehas been known, in which a supercharger is disposed rearwardly of acylinder block and air taken in from above the engine is introduced asintake air via an air intake duct to the supercharger (for example,Patent Document 1).

PRIOR ART LITERATURE

-   [Patent Document 1] JP Laid-open Patent Publication No. H02-070920

In a case where air taken in from above the engine is introduced to theengine so as to avoid interference with mounted devices as described inPatent Document 1, the length of the air intake duct is increased, and aflow velocity may be reduced due to passage loss in the air intake duct.If a flow velocity is insufficient near a suction port of thesupercharger, an efficiency of the supercharger is reduced.

SUMMARY OF THE INVENTION

In view of the aforementioned problem, an object of the presentinvention is to provide an air intake duct, of a saddle-riding typevehicle, in which a flow velocity of intake air is less likely to bereduced to enable prevention of reduction of an efficiency of asupercharger.

In order to achieve the aforementioned object, the present inventionprovides an air intake duct for supplying incoming wind as an intake airto a supercharger for a combustion engine of a saddle-riding typevehicle, which includes an air intake port defined at a front endthereof and a downstream portion defined at a rear end thereof, in whichcase a passage area is gradually reduced from the air intake port towardthe downstream portion connected to the supercharger. In the descriptionherein, “gradually reduced” means that the area is reduced as a wholeand may include a portion having a partially constant area.

According to this configuration, since the passage area of the airintake duct is gradually reduced from the air intake port toward thedownstream portion connected to the supercharger, a flow velocity of theintake air is gradually increased towards the supercharger. As a result,the flow velocity of the intake air is not reduced near a suction portof the supercharger to assuredly obtain a high efficiency of thesupercharger. Further, since the flow velocity of the intake air isgradually increased, unevenness of the flow is reduced and an air intakeefficiency is also high.

In the present invention, it is preferable that the air intake ductextends from the air intake port disposed on one side of a vehicle body,and passes laterally of one side of a cylinder block, to introduce theintake air to the supercharger disposed rearwardly of the cylinderblock. In this case, an outer side wall of the air intake port ispreferably positioned outward of an inner side wall of an outermost sideportion of the air intake duct. According to this configuration,deflection, in a vehicle widthwise direction, of flow rate of the intakeair from the air intake port toward the supercharger is reduced, andtherefore, unevenness of the flow is reduced, to enhance an air intakeefficiency.

In the present invention, a connecting portion having an increasedpassage area is preferably provided in the downstream portion. Accordingto this configuration, since intake air is accumulated in the connectingportion, intake air can be stably supplied to the supercharger.

When the connecting portion is provided, a cleaner element for cleaningthe intake air is preferably disposed in the connecting portion, and anoutlet, of the connecting portion, connected to the supercharger ispreferably set so as to have a passage area that is less than an area ofthe cleaner element. According to this configuration, a flow rate of theintake air is reduced at the connecting portion, and therefore, loss inpassing through the element is reduced. Further, since a passage area ofthe outlet of the connecting portion is set so as to be reduced, a flowrate of the intake air is increased, and therefore, reduction of anefficiency of the supercharger due to reduction of a flow velocity ofthe intake air near the suction port of the supercharger may not becaused.

In the present invention, an introduction member for introducing theintake air to a suction port of the supercharger is preferably providedinside the air intake duct. According to this configuration, since theintake air is stably introduced to the supercharger, an efficiency ofthe supercharger is enhanced.

In the present invention, it is preferable that the air intake ductforms a curve portion and is connected to the supercharger rearwardly ofthe curve portion, and the air intake duct has a transversecross-section which is shaped such that a dimension in an orthogonaldirection that is orthogonal to a radial direction of the curve portionis gradually reduced from the center of the curve portion toward anouter side, in the radial direction, of the curve portion. According tothis configuration, although a flow velocity of intake air is graduallyincreased in an air intake passage, and unevenness is likely to occursuch that concentration of intake air is increased outward due to acentrifugal force at the curved portion, since the transversecross-section of the air intake duct is shaped such that the passage isnarrowed toward the outer side, in the radial direction, of the curveportion, unevenness of the intake air distribution is reduced. As aresult, reduction of an efficiency of the supercharger can be prevented.

In the present invention, an opening edge of the air intake portpreferably extends rearwardly so as to be tilted outward in a vehiclewidthwise direction in a planar view. According to this configuration, astreamline shape of the vehicle body can be maintained while an openingarea of the air intake port can be increased.

Any combination of at least two constructions, disclosed in the appendedclaims and/or the specification and/or the accompanying drawings shouldbe construed as included within the scope of the present invention. Inparticular, any combination of two or more of the appended claims shouldbe equally construed as included within the scope of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

In any event, the present invention will become more clearly understoodfrom the following description of preferred embodiments thereof, whentaken in conjunction with the accompanying drawings. However, theembodiments and the drawings are given only for the purpose ofillustration and explanation, and are not to be taken as limiting thescope of the present invention in any way whatsoever, which scope is tobe determined by the appended claims. In the accompanying drawings, likereference numerals are used to denote like parts throughout the severalviews, and:

FIG. 1 is a side view of a motorcycle, which is one type of asaddle-riding type vehicle, having an air intake duct mounted thereinaccording to a first embodiment of the present invention;

FIG. 2 is a perspective view of a main portion of the motorcycle asviewed from diagonally above a rear portion thereof;

FIG. 3 is a side view of a main portion of the motorcycle;

FIG. 4 is a side view of the air intake duct;

FIG. 5 is a rear view of the air intake duct;

FIG. 6 is a plan view of the motorcycle;

FIG. 7 is a cross-sectional view as taken along a line VII-VII in FIG.3;

FIG. 8A is a cross-sectional view of another example of the air intakeduct;

FIG. 8B is an enlarged view of a portion VIIIb of FIG. 8A;

FIG. 8C illustrates another example of the portion VIIIb;

FIG. 9 is a plan view of a main portion of a motorcycle, which is onetype of a saddle-riding type vehicle, having an air intake duct mountedthereto according to a second embodiment of the present invention;

FIG. 10 is a plan view of a main portion of a motorcycle, which is onetype of a saddle-riding type vehicle, having an air intake duct mountedthereto according to a third embodiment of the present invention;

FIG. 11 is a perspective view of a main portion of the motorcycle asviewed from diagonally above a front portion thereof; and

FIG. 12 is a cross-sectional view of an example of an air intake ductthat is different from those of the first to the third embodiments.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a preferred embodiment of the present invention will bedescribed with reference to the accompanying drawings. The terms “left”and “right” used in the description in this specification are relativeterms denoting respective position and/or directions relative to amotorcycle driver maneuvering the motorcycle to travel forwards.

FIG. 1 is a side view of a motorcycle, which is one type of asaddle-riding type vehicle, having an air intake duct mounted thereinaccording to a first embodiment of the present invention. A motorcycleframe structure FR for the motorcycle includes a main frame 1 whichforms a front half of the motorcycle frame structure FR, and a seat rail2, mounted on a rear portion of the main frame 1, which forms a rearhalf of the motorcycle frame structure FR. The main frame 1 has a frontend provided with a head pipe 4, and a front fork 8 is rotatablysupported by this head pipe 4 through a not-illustrated steering shaft.A front wheel 10 is fitted to the front fork 8, and a steering handle 6for operation is fixed to an upper end portion of the front fork 8.

Meanwhile, a swingarm 12 is supported by a rear end portion of the mainframe 1, which is a lower intermediate portion of the motorcycle framestructure FR, for movement in up and down direction about a pivot pin16. A rear wheel 14 is rotatably supported by a rear end portion of theswingarm 12. A combustion engine E is fitted to the lower portion of themain frame 1. A radiator 13 for engine cooling water is disposedforwardly of the combustion engine E. Rotation of the combustion engineE is transmitted to a transmitting member (not shown) such as a chain,to drive the rear wheel 14 via the transmitting member. A side stand 17is supported at the rear end of the main frame 1 on the left side of amotorcycle body, such that the side stand 17 can be lifted up and down.

A fuel tank 15 is disposed on an upper portion of the main frame 1 and adriver's seat 18 and a fellow passenger's seat 20 are supported by theseat rail 2. Further, a front fairing or cowl 22 made of a resinousmaterial is mounted on a front portion of the motorcycle body so as tocover a front portion of the head pipe 4. The front cowl 22 has an airintake port 24 through which air is taken from the outside into thecombustion engine E.

The air intake port 24 is opened forward of the motorcycle body, wherebyan amount of intake air to the combustion engine E can be increased byutilizing wind pressure of incoming wind A. The air intake port 24 isdisposed on the front surface of the front cowl 22, so as to be disposedat the front end portion of the front cowl 22 at which a dynamicpressure of the incoming wind is maximum. Thus, lateral projection ofthe air intake port 24 from the front cowl 22 is reduced as compared toa case where the air intake portion is disposed so as to project on theside portion of the front cowl 22. As a result, the air intake port 24may not be outstanding, to improve an outer appearance of themotorcycle.

The combustion engine E is a parallel multicylinder combustion engine,such as a parallel four cylinder, four-stroke cycle combustion engine,and has an engine rotary shaft 26 that extends in a motorcycle widthwisedirection or a left-right direction. A type of the combustion engine Eis not necessarily limited thereto. The combustion engine E includes: acrankcase 28 for supporting the engine rotary shaft 26; a cylinder block30 connected to an upper portion of the crankcase 28; a cylinder head 32connected to an upper portion of the cylinder block 30; a head cover 32a mounted on an upper portion of the cylinder head 32; and an oil pan 34mounted to a lower portion of the crankcase 28.

The cylinder block 30 and the cylinder head 32 are slightly tiltedforward. Specifically, a piston axis line of the combustion engine Eextends upward so as to be tilted forward. Air intake ports 47 areprovided in the rear portion of the cylinder head 32. Four exhaust pipes36, connected to exhaust ports in a front surface of the cylinder head32, are merged together below the combustion engine E, and are connectedto an exhaust muffler 38 disposed at the right side of the rear wheel14. A supercharger 42 is disposed rearwardly of the cylinder block 30 anabove the crankcase 28.

The supercharger 42 compresses the outside air taken in through asuction port 46, and discharges, after enhancing the pressure of theair, the air through a discharge port 48, to supply the air to thecombustion engine E. Thus, an amount of intake air to be supplied to thecombustion engine E can be increased. The supercharger 42 includes: asupercharger rotary shaft 44 that extends in the motorcycle widthwisedirection; the suction port 46 disposed above the rear portion of thecrankcase 28 so as to be opened leftward; and the discharge port 48disposed at the intermediate portion, in the motorcycle widthwisedirection, of the combustion engine E in the rear of the superchargerrotary shaft 44 so as to be opened upward.

As shown in FIG. 2, the supercharger 42 includes: an impeller 50 forpressurizing intake air; an impeller housing 52 for covering theimpeller 50; a transmission mechanism 54 for transmitting power of thecombustion engine E to the impeller 50; and a transmission mechanismhousing 56 for covering the transmission mechanism 54. The transmissionmechanism 54 is implemented as, for example, a speed increaser or aspeed increasing gear. The transmission mechanism 54 and an air cleaner40 are disposed so as to interpose the impeller housing 52 therebetweenin the motorcycle widthwise direction. The impeller housing 52 isconnected to the transmission mechanism housing 56 and the air cleaner40 by means of not-illustrated bolts. However, the structure of thesupercharger 42 is not necessarily limited to that of the presentembodiment.

The suction port 46 of the supercharger 42 is disposed inward of theleft side surface of the cylinder block 30 in the motorcycle widthwisedirection. Thus, the supercharger 42 does not project laterally outwardof the left side surface of the cylinder block 30, and an assembly ofthe combustion engine E and the supercharger 42 is made compact. Acleaner outlet 62 of the air cleaner 40 is connected to the suction port46, and an air intake duct 70, through which the incoming wind A(FIG. 1) flowing forwardly of the cylinder block 30 is introduced to thesupercharger 42, is connected to a cleaner inlet 60 of the air cleaner40 from the outer side in the motorcycle widthwise direction. Thecleaner inlet 60 and a discharge port 70 b of the air intake duct 70 areconnected to each other by connecting flanges 63, 64 provided on outerperipheries thereof, respectively, by means of a plurality of bolts 55.

A cleaner element 69 for cleaning intake air I is interposed between theconnecting flanges 63 and 64 that form an upstream end portion of theair cleaner 40. A cleaner body 65 forming a cleaning chamber is provideddownstream of the connecting flanges 63, 64. The intake air I, whenpassing through the cleaner element 69, is cleaned and flow-regulated.That is, the cleaner element 69 also functions as a suppressing memberfor suppressing unevenness of the intake air I in an air intake passage77 (FIG. 7) provided upstream of the suction port 46 of the supercharger42. As such a suppressing member other than the cleaner element 69 ofthe air cleaner 40, a perforated metal may be used.

An air intake chamber 74 is disposed between the discharge port 48 ofthe supercharger 42 and the air intake ports 47 of the combustion engineE shown in FIG. 1. The air intake chamber 74 accumulates intake air tobe supplied to the air intake ports 47 from the supercharger 42. The airintake chamber 74 is disposed above the supercharger 42 and rearwardlyof the cylinder block 30. As shown in FIG. 2, the discharge port 48 ofthe supercharger 42 is connected to the intermediate portion, in themotorcycle widthwise direction, of the air intake chamber 74. Thus, theintake air evenly flows from the supercharger 42 through the air intakechamber 74 to a plurality of the air intake ports 47.

As shown in FIG. 1, a throttle body 76 is disposed between the airintake chamber 74 and the cylinder head 32. A fuel is injected into theintake air within the throttle body 76 to produce an air/fuel mixture,and the mixture is supplied into a cylinder. The fuel tank 15 isdisposed above the air intake chamber 74 and the throttle body 76.

The air intake duct 70 forms the air intake passage 77 (FIG. 7) throughwhich intake air is supplied to the supercharger 42, and the air intakeduct 70 is disposed on the left side of the motorcycle body, on whichthe side stand 17 is also disposed. A passage area of the air intakepassage 77 is set so as to be gradually reduced, from the air intakeport 24 disposed in the front end, toward a downstream portion 70 c(FIG. 2) that is disposed in the rear end to connect to the supercharger42. In the description herein, “gradually reduced” means that thepassage area may be reduced as a whole and may include a portion havinga partially constant passage area.

The passage area of the air intake passage 77 is set so as to be variedby at least one of a dimension of a cross-section of the air intake duct70 in the vertical direction and a dimension thereof in the widthwisedirection being gradually reduced towards the rearward. In the presentembodiment, as shown in FIG. 4 and FIG. 5, both a dimension H in thevertical direction and a dimension W in the widthwise direction arevaried.

Since the cross-sectional area of the air intake duct 70 is graduallyreduced towards the rearward, a portion of the air intake duct 70 shownin FIG. 1, which is exposed to the outside of the motorcycle body in therear portion, can be reduced. In other words, a portion of the rearportion of the air intake duct 70, which portion is near a leg of arider, can be reduced. Thus, interference between the air intake duct 70and a leg of a rider can be prevented. As a result, a driving positionof the rider can be prevented from being tight or a rider having a largebody can easily drive the motorcycle.

The air cleaner 40 forms a connecting portion 67 that is provided in thedownstream portion 70 c of the air intake duct 70 and that has anincreased passage area, or a passage area is creased than an upstreamportion thereof. The upstream end portion of the connecting portion 67is formed by the connecting flanges 63, 64 and the connecting portionbody of the connecting portion 67 is formed by the cleaner body 65. Thecleaner outlet 62 in the cleaner body 65 acting as the connectingportion body is connected to the supercharger 42 and is set so as tohave a passage area that is less than an area of the cleaner inlet 60 atwhich the cleaner element 69 is disposed.

As is clear from FIG. 1, the air intake duct 70 has a lowermost portion70 d at its intermediate portion in a flow direction in which the intakeair I flows. Thus, by providing the lowermost portion 70 d at theintermediate portion, the air intake duct 70 can be V-shaped in thelateral view. However, the shape of the air intake duct 70 is notnecessarily limited to such a V-shape, and may be a straight shape.Specifically, as indicated by a double dotted line in FIG. 4, an airintake duct 70L may extend so as to be gradually tilted downward towardsthe rear in the lateral view. Thus, resistance or drag of intake air canbe reduced.

The air intake duct 70 includes a ram duct unit 80 on the upstream sideand an intake duct unit 82 on the downstream side as shown in FIG. 3.The ram duct unit 80 is supported by the main frame 1 so as to face afront end opening 70 a thereof shown in FIG. 1 toward the air intakeport 24 of the front cowl 22, and thus, pressure of air introducedthrough the opening 70 a is enhanced by the ram effect. A front endportion 82 a of the intake duct unit 82 is connected to a rear endportion 80 b of the ram duct unit 80 shown in FIG. 3. The discharge port70 b that is a rear end portion of the intake duct unit 82 is connectedto the cleaner inlet 60 (FIG. 2) of the air cleaner 40.

The ram duct unit 80 is disposed frontwardly the head pipe 4, and is,for example, fixed to the front cowl 22 (FIG. 1). The inside of the headpipe 4 may serve as a portion of the air intake passage 77 (FIG. 7) inthe ram duct unit 80.

The front end opening 70 a of the ram duct unit 80 acts as theintroduction port 70 a of the air intake duct 70. As shown in FIG. 6,the introduction port 70 a (the air intake port 24) of the air intakeduct 70 is laterally elongated, and is disposed frontwardly of the headpipe 4 and in the left side portion of the motorcycle body. Thus, adistance from the introduction port 70 a to the supercharger 42 as shownin FIG. 6 can be reduced and a curvature of the air intake duct 70 canbe reduced, as compared to a case where the introduction port 70 a isdisposed at the intermediate portion in the motorcycle widthwisedirection.

The opening edge of the air intake port 24 extends rearwardly so as tobe tilted outward in the motorcycle widthwise direction, in a planarview. An outermost side end 24 a, in the motorcycle widthwise direction,of the air intake port 24 is disposed outward of an inner side surface70 i, in the motorcycle widthwise direction, of an outermost curvedportion of the air intake duct 70. Thus, intake air is easily introducedalso to an outer side portion inside the air intake duct 70 that iscurved outward.

In the present embodiment, the introduction port 70 a and the dischargeport 70 b of the air intake duct 70 are disposed inward of the outerside surfaces of the motorcycle frame structure FR and the combustionengine E. On the other hand, the intermediate portion, in the front-reardirection, of the air intake duct 70 is disposed outward of the outerside surfaces of the motorcycle frame structure FR and the combustionengine E.

However, the introduction port 70 a of the air intake duct 70 may bedisposed anywhere on the front surface, and for example, may be disposednear the front end portion that is a foremost portion at theintermediate portion, in the motorcycle widthwise direction, of thefront cowl 22 shown in FIG. 6. In this case, the incoming wind A in ahigh pressure state can be introduced to the supercharger 42.

The air intake duct 70 is disposed outward of the main frame 1 in thewidthwise direction. Thus, interference between the air intake duct andthe main frame 1 can be prevented while increase of the dimension, inthe widthwise direction, of the main frame 1 can be suppressed. In thepresent embodiment, the main frame 1 is an outermost side portion of themotorcycle body except for the air intake duct 70. A knee grip portion75 is formed frontwardly of the driver's seat 18 so as to have adimension reduced in the motorcycle widthwise direction, and the airintake duct 70 projects laterally outward of the main frame 1 in frontof the knee grip portion 75. The knee grip portion 75 is formed in therear portion of a tank side cover 79 that covers the outside of a lowerportion of the fuel tank 15 shown in FIG. 1.

The intake duct unit 82 smoothly connects between the ram duct unit 80and the supercharger 42. The intake duct unit 82 is tilted downwardtowards the rear from the ram duct unit 80, while bulging leftward, andpasses laterally of the cylinder block 30. That is, as shown in FIG. 6,the intake duct unit 82 extends so as to be curved outward of themotorcycle body towards the rear, and connects to the supercharger 42. Aportion of the inner side surface 70 i of the air intake duct 70,specifically, an inner side surface of the curved portion extendsoutward towards the rear. Since the inner side surface of the curvedportion is tiled outward as described above, reduction of a flowvelocity in the inner side portion of the air intake duct 70 can besuppressed.

Specifically, the air intake duct 70 is curved in the vertical directionand the widthwise direction. That is, the air intake duct 70 extends soas to be curved downward towards the rear, and extends so as to becurved outward in the motorcycle widthwise direction. The air intakeduct 70 is curved so as to have a complicated curved shape due tovarious causes. Specifically, the air intake duct 70 is curved so as notto interfere with a leg of a rider as shown in FIG. 1, curved such thatthe incoming wind A that has passed through the radiator 13 is notblocked by the air intake duct 70, curved so as not to interfere withthe steering handle 6 and the front fork 8 in steering operation, orcurved such that the intake air I smoothly flows through the air intakeduct 70.

The air intake duct 70 shown in FIG. 1 passes below the tip end portionof the steering handle 6 and above the radiator 13 and passes laterallyby the outer side of the front fork 8 in a region frontwardly of thecombustion engine E, as viewed from the side thereof. Specifically, theair intake duct 70 passes below a region in which the steering handle 6performs rotational movement. Thus, interference between the air intakeduct 70 and the steering handle 6 can be prevented. Further, the airintake duct 70 passes above a space rearwardly of the radiator 13. Thus,the space rearwardly of the radiator 13 can be opened in the motorcyclewidthwise direction, and as a result, the incoming wind A that passesthrough the radiator 13 is smoothly discharged. The lower end of the airintake duct 70 near the radiator 13 is preferably disposed above arotary shaft of a fan (not shown) of the radiator 13, and morepreferably disposed above the upper end of the fan (not shown). Further,the air intake duct 70 passes above the exhaust pipes 36 frontwardly ofthe combustion engine E.

Further, the air intake duct 70 extends rearwardly below the fuel tank15 and laterally of the outer side of the main frame 1. Thus, a capacityof the fuel tank 15 can be ensured, and interference between the airintake duct 70 and the main frame 1 can be avoided. Further, forexample, a wiring harness or the piping portion fixed to the main frame1 can be hidden by the air intake duct 70.

Further, in a region lateral to the combustion engine E, the air intakeduct 70 passes above the crankshaft 26, specifically, passes above agenerator cover 29 mounted to a left side portion of the crankcase 28and below the upper surface of the cylinder head 32 and the throttlebody 76, as viewed from the side. A portion of the side surface and aportion of the upper surface of the intake duct unit 82 are exposed tothe outside. Further, the rear end of a portion, of the air intake duct70, which projects from the main frame 1 is disposed, below the knee Kof a rider seated on the motorcycle and forwardly of a below-kneeportion KU.

Thus, since the air intake duct 70 extends below the upper surface ofthe cylinder head 32 in the region lateral to the combustion engine, theair intake duct 70 need not be sharply curved downward so as to connectto the cleaner inlet 60 (FIG. 2). Accordingly, a radius of curvature ofthe air intake duct 70 can be increased, as compared to a case where theair intake duct 70 passes above the upper surface of the cylinder head32. Further, since the air intake duct 70 passes below the throttle body76 in the region lateral to the combustion engine E, interference withcomponents, such as a throttle valve driving mechanism or a sensor,provided in opposite side portions of the throttle body 76 can beprevented, and the air intake duct 70 can be extended to the inner sideportion in the motorcycle widthwise direction to the maximum extent.Further, since the air intake duct 70 extends above the generator cover29, interference between the air intake duct 70 and the generator cover29 can be prevented.

As shown in FIG. 7, the air intake duct 70 is dividable into two partsof a left part and a right part, and includes an inner half part 90 andan outer half part 92. The inner half part 90 and the outer half part 92are each formed so as to have a U-shaped cross-section. Thus, molding isfacilitated and strength can be assuredly obtained. Specifically, theouter half part 92 is formed such that a cross-sectional shape thereofperpendicular to the front-rear direction is a U-shape that is openedinward in the motorcycle widthwise direction. On the other hand, theinner half part 90 is formed such that a cross-sectional shape thereofperpendicular to the front-rear direction is a U-shape that is openedoutward in the motorcycle widthwise direction. The cross-sectional shapeof each of the outer half part 92 and the inner half part 90 is notnecessarily limited to a U-shape. For example, the outer half part 92and the inner half part 90 may be each formed so as to have an L-shapedcross-section, or the inner half part 90 may be formed so as to have anI-shaped cross-section.

A material or a surface treatment is different between the outer halfpart 92 and the inner half part 90. For the outer half part 92, such amaterial or a surface treatment as to improve an outer appearance isnecessary. On the other hand, for the inner half part 90, conditionssuch as strength or production cost, other than an outer appearance, aremore important. When a material or a surface treatment is made differenttherebetween, requirements for each of the outer half part 92 and theinner half part 90 can be fulfilled. In the present embodiment, theouter half part 92 is formed by a material obtained by coloring an ABSresin (acrylonitrile-butadiene-styrene copolymer synthetic resin), andthe inner half part 90 is formed from a polypropylene (PP). However, thematerial of each of the outer half part 92 and the inner half part 90 isnot necessarily limited to the above materials.

On the surface (the inner side surface of the intake duct 70) of theinner half part 90, a fixing portion 100 for attaching the air intakeduct 70 to the main frame 1 is formed. The ram duct unit 80 shown inFIG. 6 is fixed to the front cowl 22, so that the front portion of theair intake duct 70 is supported by the motorcycle body. The rear portionof the air intake duct 70 is connected to the air cleaner 40 by means ofthe bolts 55 (FIG. 2), and thus, is supported by the motorcycle body.The number of the fixing portions 100 shown in FIG. 7 may be plural asnecessary, thereby supporting the intermediate portion, in thefront-rear direction, of the air intake duct 70 by the motorcycle body.

The outer half part 92 and the inner half part 90 are joined to eachother in a state where an inner side end portion of the outer half part92 and an outer side end portion of the inner half part 90 face or butteach other. The inner half part 90 and the outer half part 92 are joinedto each other by means of, for example, melt-bonding or adhesion. Theinner side end portion, on the upper side, of the outer half part 92has: a joining portion 92 a that butts the outer side end portion of theinner half part 90; and a projection 92 b formed above the joiningportion 92 a and projecting toward the inner side of the motorcycle bodybeyond the joining portion 92 a. An upper dividing face 94 between theinner and outer half parts 90 and 92 is disposed inward of theintermediate position, in the widthwise direction, of the air intakeduct 70. A lower dividing face 96 between the inner and outer half parts90 and 92 is disposed outward of the upper dividing face 94 in thewidthwise direction.

As shown in FIG. 8A, the lower dividing face 96 may be disposed outwardof the intermediate portion, in the widthwise direction, of the airintake duct 70. Further, as shown in FIG. 8B, a joining portion betweenthe inner half part 90 and the outer half part 92 may be formed suchthat, a protrusion 93 of the inner half part 90 may be fitted into arecess 95 of the outer half part 92. Thus, strength at the joiningportion is enhanced. Further, as shown in FIG. 8C, a return portion 97for preventing disengagement in the widthwise direction at the joiningportion may be provided in one of the inner and outer half parts 90 and92.

A transverse cross-sectional shape of the intake duct unit 82 formingthe air intake passage 77 shown in FIG. 7 is a rectangular shape thathas a longitudinal axis extending almost in the vertical direction. Inthe curved portion, a transverse cross-section of the intake duct unit82 is formed so as to have such a shape that a dimension D1 in theorthogonal direction (dimension in the vertical direction in thetransverse cross-section) orthogonal or perpendicular to the radialdirection of the curve portion is gradually reduced from the center sideof the curve portion (the inner side of the motorcycle body) toward theouter side in the motorcycle widthwise direction. The latter sidecorresponds to the outer side, in the radial direction, of the curveportion. The above identified orthogonal direction is perpendicular tothe motorcycle widthwise direction. In the description herein,“gradually reduced” means that the dimension may be reduced as a wholeand may include a portion having a partially constant dimension. Theintake duct unit 82 may have, for example, a D-shaped cross-section, aV-shaped cross-section, or a trapezoidal cross-section in which an outerside edge projects or bulges in the radial direction (outward in themotorcycle widthwise direction) of the curve portion so as to form anarcuate shape.

Such a dimension-reducing shape that has the dimension D1, in theorthogonal direction, which is gradually reduced towards the outside,may be formed over the entirety of the duct, and however, such adimension-reducing shape may be preferably formed in at least a regionin which the curvature is reduced, or may be formed only in such aregion in which the curvature is reduced. Specifically, the duct rearportion 70 b, at which the flowing direction of the intake air Isubstantially changes from the front-rear direction to the widthwisedirection in the air intake duct 70, is preferably formed as adimension-reducing shape.

Although, in the present embodiment, the air intake duct 70 is curved inthe widthwise direction, shape of the air intake duct 70 is notnecessarily limited thereto. For example, where the air intake duct 70passes above the combustion engine E in the front-rear direction and iscurved in the vertical direction, the air intake duct 70 is formed assuch a dimension-reducing shape that the dimension in the widthwisedirection in the curved portion is reduced upwardly.

The transverse cross-section is shaped, in the curved portion of the airintake passage 77 in the intake duct unit 82, such that a maximum valueD1max of the dimension D1 in the orthogonal direction is set so as to begreater than a maximum value D2max of a dimension D2 in the radialdirection (dimension in the widthwise direction) along the radialdirection of the curve portion (D1max>D2max). In other words, thetransverse cross-section of the intake duct unit 82 is formed by a ductinner side line 84 on the center side of the curve portion, a duct outerside line 86 on the outer side, in the radial direction, of the curveportion, and duct connecting lines 88, 88 that connect between the ductinner side line 84 and the duct outer side line 86, in which adimension, in the orthogonal direction, of the duct outer side line 86is set so as to be less than that of the duct inner side line 84. Thus,since the dimension in the vertical direction is set so as to be greaterthan the dimension in the motorcycle widthwise direction, extension ofthe air intake duct 70 in the motorcycle widthwise direction can beprevented, while an area of the passage can be increased. The dimensionD1 in the orthogonal direction and the dimension D2 in the radialdirection correspond to the dimension H, in the vertical direction, ofthe cross-section of the air intake duct 70, and the dimension W, in thewidthwise direction, of the cross-section of the air intake duct 70,respectively, as shown in FIG. 4 and FIG. 5.

A chamfered portion 89 is formed at the corner, on the upper-outer side,of the cross-section of the air intake duct 70 having almost arectangular shape. Thus, reduction of a flow velocity at the corner canbe prevented. Further, in the cross-section of the air intake duct 70,an inner surface of the inner half part 90 and an inner surface of theouter half part 92 are formed so as to be flush with each other. Also bythis configuration, a drag in the flow passage can be reduced.

A part of the outer side surface of the air intake duct 70 shown in FIG.1 is exposed to the outside in the motorcycle widthwise direction. Inthe present embodiment, a portion, of the air intake duct 70, exposed tothe outside in the motorcycle widthwise direction passes outside themotorcycle frame structure FR in the motorcycle widthwise direction nearthe steering handle 6. Specifically, the portion exposed to the outsideextends from a vicinity of the steering handle 6 to a vicinity of therear portion of the combustion engine E in the front-rear direction.Thus, visibility of the side surface of the air intake duct 70 from arider or the outside is enhanced. Since no partition line or dividingline along the dividing face 94 or 96 in FIG. 7 is formed on the sidesurface of the air intake duct 70 as described above, a partition lineis difficult to see from a rider or the outside. Thus, an outerappearance of the motorcycle is improved.

As shown in FIG. 6, a part of the upper surface of the air intake duct70 is also exposed upwardly of the motorcycle body. In the presentembodiment, a portion, of the air intake duct 70, exposed at upwardly ofthe motorcycle body passes outside the motorcycle frame structure FR inthe motorcycle widthwise direction near the steering handle 6.Specifically, the portion exposed upwardly extends from the vicinity ofthe steering handle 6 to the vicinity of the rear portion of thecombustion engine E in the front-rear direction. Thus, visibility of theupper surface of the air intake duct 70 from a rider or the outside ofthe motorcycle is enhanced. Since the partition line 94 on the uppersurface of the air intake duct 70 is formed on the inner side asdescribed above, the partition line 94 is difficult to see from a rideror the outside. Thus, an outer appearance of the motorcycle is improved.

Where the air intake duct 70 is partially hidden on the inner side inthe motorcycle widthwise direction by the motorcycle frame structure orthe cowl in a planer view, the partition line may be displaced outwardin the motorcycle widthwise direction within a range in which thepartition line is hidden by the motorcycle frame structure or the cowlin the planer view. Thus, an outer appearance is maintained excellent,and enhancement of strength is facilitated. For example, the partitionline may be extended along a boundary of the region hidden by themotorcycle frame structure or the cowl.

When the crankshaft 26 shown in FIG. 1 rotates, power of the combustionengine E is transmitted through the transmission mechanism 54 to thesupercharger 42, to actuate the supercharger 42. When the motorcycletravels, the incoming wind A is introduced from the air intake port 24and flows in the ram duct unit 80 and then the intake duct unit 82.Then, the incoming wind A is cleaned by the air cleaner 40, and isintroduced into the supercharger 42. The incoming wind A having beenintroduced into the supercharger 42 is pressurized by the supercharger42 and is supplied through the air intake chamber 74 and the throttlebody 76 into the combustion engine E. By a synergistic effect of the rampressure and pressurizing by the supercharger 42 as described above,high-pressure intake air can be supplied to the combustion engine E.

In the above structure, the passage on the outer side, in the radialdirection, of the curve portion is narrower than the passage on theinner side, in the radial direction, of the curve portion in the airintake passage 77 shown in FIG. 7. Therefore, the intake air I is lesslikely to be deflected outward in the radial direction of the curveportion due to a centrifugal force, and a flow of the intake air I ismade even in the air intake passage 77. Thus, the passage is connectedto the suction port 46 of the supercharger 42 such that the flow of theintake air I is made even, and therefore, reduction of an efficiency ofthe supercharger 42 can be prevented.

Further, the transverse cross-section of the air intake passage isshaped such that a maximum dimension D1max in the orthogonal directionis set so as to be greater than the maximum dimension D2max in theradial direction of the curve portion. Thus, reduction of unevenness inthe flow velocity is facilitated as compared to a case where the maximumdimension in the radial direction is greater than the maximum dimensionin the orthogonal direction.

As shown in FIG. 1, since the incoming wind A is taken in through theair intake port 24 that is opened forward, a flow velocity is increasedto obtain a high dynamic pressure, while influence of a centrifugalforce is likely to be exerted. However, as described above, sinceunevenness of the intake air I is reduced, reduction of an efficiency ofthe supercharger 42 can be prevented.

The supercharger 42 shown in FIG. 2 is disposed rearwardly of thecylinder block 30 of the combustion engine E, and the air intake duct 70extends so as to be curved from the front of the cylinder block 30toward the left side of the motorcycle body, and pass laterally of theleft side of the cylinder block 30, to be connected to the supercharger42. Thus, since the air intake duct 70 is curved in the lateraldirection, the dimension, in the vertical direction, of the motorcyclecan be reduced, as compared to a case where the air intake duct passesabove the cylinder block 30.

The air intake port 24 of the air intake duct 70 shown in FIG. 6 isdisposed on the left side of the motorcycle body, and the outermost sideend 24 a, in the motorcycle widthwise direction, of the air intake port24 is disposed outward of the inner side surface 70 i, in the motorcyclewidthwise direction, of an outermost curved portion of the air intakeduct 70. Accordingly, since the curve of the air intake duct 70 can bereduced, deflection of the flow rate of the intake air I in themotorcycle widthwise direction can be reduced, and also a dimension ofthe motorcycle in the motorcycle widthwise direction can be reduced.

The suction port 46 of the supercharger 42 shown in FIG. 2 is disposedinward of the left side surface of the combustion engine E in themotorcycle widthwise direction. In a case where the suction port 46 isdisposed inward of the combustion engine E, the supercharger 42 does notproject laterally outward of the left side surface of the combustionengine E, and therefore, an assembly of the combustion engine E and thesupercharger 42 can be made compact, while a curvature of the air intakeduct 70 may be increased, and a centrifugal force may be increased.However, as described above, unevenness of the intake air I is reduced,and therefore, reduction of an efficiency of the supercharger 42 can beprevented. Further, deflection, in the motorcycle widthwise direction,of the intake air I introduced from the air intake port 24 toward thesupercharger 42 is reduced, and therefore, unevenness of the flow isreduced to enhance an air intake efficiency.

Further, the passage area of the air intake passage is set so as to begradually reduced from the upstream side toward the downstream side.Thus, a flow velocity of the intake air I is gradually increased towardsthe downstream side. As a result, a flow velocity is not reduced nearthe suction port 46 of the supercharger 42, and a high efficiency of thesupercharger 42 can assuredly be obtained. Further, since the flowvelocity of the intake air I is gradually increased, unevenness of theflow is reduced, and therefore, an air intake efficiency is also high.In this case, although an influence of a centrifugal force is likely tobe exerted near the suction port 46 of the supercharger 42 at which aflow velocity is high, as described above, unevenness of the intake airI is reduced, and therefore, reduction of an efficiency of thesupercharger 42 can be prevented.

The cleaner element 69 is disposed upstream of the suction port 46 ofthe supercharger 42 as shown in FIG. 2. Since unevenness of the intakeair I in the air intake passage is reduced by the cleaner element 69,unevenness of the intake air I introduced into the supercharger 42 isreduced, and therefore, reduction of an efficiency can effectively beprevented.

As shown in FIG. 7, the air intake duct 70 is dividable into two partsof the left part and the right part, and therefore, the air intake duct70 can be formed by molding. As a result, even in a case where the airintake duct 70 is curved in the vertical direction and in the widthwisedirection, the air intake duct 70 can be easily formed.

Although a part of the side surface of the air intake duct 70 shown inFIG. 6 is exposited to the outside, no partition line is provided on theside surface. Therefore, even in a case where the air intake duct 70 isexposed, the outer appearance is not degraded.

The upper dividing face 94 of the air intake duct 70 shown in FIG. 7 isdisposed inward of the intermediate position, in the motorcyclewidthwise direction, of the air intake duct 70. Therefore, even in acase where a part of the upper surface of the air intake duct 70 isexposed to the outside, the inner half part 90 is less likely to beexposed to the outside. As a result, only the outer half part 92 can beformed as a design component to improve an outer appearance, while theinner part can be produced at low cost.

As shown in FIG. 1, in a case where the air intake duct 70 passes abovethe radiator 13 or passes laterally of the outer side of the front fork8, the air intake duct 70 can be easily seen from a rider. However, asdescribed above, the upper dividing face 94 is disposed on the innerside of the motorcycle body, and therefore, the partition line may notbe outstanding.

Since the lower dividing face 96 shown in FIG. 7 is disposed outward ofthe upper dividing face 94 in the motorcycle widthwise direction, theinner half part 90 is less likely to be flattened, and therefore,rigidity of the air intake duct 70 can be enhanced.

The outer half part 92 and the inner half part 90 are joined to eachother in a state where the inner side end portion of the outer half part92 and the outer side end portion of the inner half part 90 face or butteach other. Therefore, an uneven portion may not be formed in the innerperipheral surface of the air intake duct 70, and as a result, a flow ofthe intake air I in the air intake duct 70 is not inhibited.

The inner side end portion, on the upper side, of the outer half part 92includes: the joining portion 92 a that butts the outer side end portionof the inner half part 90; and the projection 92 b formed above thejoining portion 92 a and projecting toward the inner side of themotorcycle body beyond the joining portion 92 a. Thus, the joiningportion 92 a on the upper side is hidden by the projection 92 b toimprove an outer appearance.

The fixing portion 100 for mounting the air intake duct 70 to the mainframe 1 is formed on the outer surface (inner side surface) of the innerhalf part 90. Therefore, the fixing portion 100 is prevented fromappearing in the outer appearance of the motorcycle body, and the outerappearance of the motorcycle body can be prevented from being degraded.

In a case where the air intake duct 70 shown in FIG. 1 is disposed onthe left side of the motorcycle body, on which the side stand 17 is alsodisposed, the upper dividing face 94 is easily seen when the motorcycleis at stoppage. However, since the air intake duct 70 is disposed at alower side of tilt when the motorcycle is at stoppage, the partitionline may not be outstanding.

In the downstream portion 70 c of the air intake duct 70 shown in FIG.2, the connecting portion 67 having an increased passage area isprovided. Since the intake air I is accumulated in the connectingportion 67, the intake air I can be stably supplied to the supercharger42.

Further, the cleaner element 69 is disposed at the connecting portion67, and the cleaner outlet 62 connected to the supercharger 42 at theconnecting portion 67 is set so as to have a passage area that is lessthan the area at which the cleaner element 69 is arranged. As a result,a velocity of the intake air I is reduced at the air cleaner 40, andtherefore, loss in passing through the cleaner element 69 is reduced.Further, since a passage area of the cleaner outlet 62 of the aircleaner 40 is set so as to be reduced, a velocity of the intake air I isincreased at the cleaner outlet 62, and therefore, reduction of anefficiency of the supercharger 42 due to reduction of a flow velocity ofthe intake air I near the suction port 46 of the supercharger 42 can notbe avoided.

The opening edge of the air intake port 24 shown in FIG. 6 is tiledoutward in the motorcycle widthwise direction towards the rear in aplaner view. Thus, a streamline shape of the motorcycle body ismaintained while an opening area of the air intake port 24 can beincreased.

Since the air intake duct 70 shown in FIG. 1 passes laterally of theleft side of the cylinder block 30, a wide space can be assuredlyobtained above the combustion engine E, to improve freedom in designing.Further, since the air intake duct 70 extends frontwardly of thesteering handle 6 and then, above the radiator 13, interference with theradiator 13 can be avoided. As a result, deterioration in performance ofthe radiator can be prevented.

Further, since the air intake duct 70 passes below the tip end portionof the steering handle 6, interference of the air intake duct 70 withthe steering handle 6 that is performing rotational movement can beprevented.

The air intake duct 70 projects laterally outward of the main frame 1frontwardly of the knee grip portion 75 shown in FIG. 6, and the rearend of a portion, of the air intake duct 70, which projects outward ofthe main frame 1 is positioned below the knee K of a rider seated on themotorcycle and frontwardly of the below-knee portion KU as viewed fromthe side thereof, as shown in FIG. 1. Thus, interference of the airintake duct 70 with the knee K of a rider can be prevented.

Further, since the air intake duct 70 has the lowermost portion 70 d atits intermediate portion in the flow direction, drainage can beperformed in the lowermost portion 70 d.

FIG. 9 is a plan view of a main portion of a motorcycle, which is onetype of a saddle-riding type vehicle, having an air intake duct mountedthereto according to a second embodiment of the present invention. Anair intake duct 70A of the second embodiment includes, in addition tothe air intake port 24 on the left side that is one side of themotorcycle body, an additional air intake port 25 on the right side thatis the other side of the motorcycle body. Further, a cleaner element 69Afor cleaning intake air is incorporated in a connecting portion betweena ram duct unit 80A and the intake duct unit 82. Therefore, thedownstream portion 70 c of the air intake duct 70A has no cleanerelement (air cleaner). The other structures are the same as those of thefirst embodiment.

According to the second embodiment, since the additional air intake port25 is provided, an amount of intake air of the incoming wind A isincreased. Further, since the cleaner element 69A is disposed on theupstream side of the air intake passage having an increased passagearea, the intake air I passes through the cleaner element 69A a low flowvelocity. Thus, loss in passing through the cleaner element 69A can bereduced.

FIG. 10 is a plan view of a main portion of a motorcycle, which is onetype of a saddle-riding type vehicle, having an air intake duct mountedthereto according to a third embodiment of the present invention, andFIG. 11 is a perspective view thereof. An air intake duct 70B of thethird embodiment has an introduction port 70 a disposed at theintermediate position, in the motorcycle widthwise direction, of thefront end of the motorcycle body. Further, the intake duct unit 82 isimplemented as a cylindrical pipe, and has such a straight shape as tosmoothly extend downward towards the rear as viewed from the side.Therefore, the air intake duct 70B of the third embodiment does notinclude the lowermost portion 70 d at the intermediate portion in thefront-rear direction as in the first embodiment. However, the air intakeduct 70B has a tilted portion 70 e that is formed at the front portionso as to be raised upward towards the rear. Thus, water contained in theintake air I can be reduced. The other structures are the same as thoseof the first embodiment. Also in the third embodiment, the same effectas that of the first embodiment is obtained.

In each embodiment described above, each of the air intake ducts 70,70A, 70B may have thereinside an introduction member 150 for introducingthe intake air I to the suction port 46 of the supercharger 42. Theintroduction member 150 is implemented as, for example, a guide plate152 integrated with an inner surface of the air intake duct 70 as shownin FIG. 12. When the introduction member 150 is provided, the intake airI is stably introduced to the supercharger 42, and therefore, anefficiency of the supercharger 42 is enhanced.

Further, the air intake duct 70 may be fixed to the motorcycle body bymeans of flange portions formed on the front end portion and the rearend portion. Each flange portion has a bolt insertion hole into which abolt is inserted from the outer lateral side. Each flange portion may beformed on either of the inner half part and the outer half part.Further, for example, the rear-side flange portion may be formed in theinner half part and the front-side flange portion may be formed in theouter half part. In this case, the flange portion formed on the outerhalf part is preferably disposed at such a position as to be hidden fromthe outside by the front cowl. Further, in a case where the flangeportion is formed on the outer half part, when the flange portion isformed in the lower portion of the outer half part, the flange portionmay not be outstanding. When each of the inner half part and the outerhalf part has the flange portion formed thereon, a supporting strengthat a joining portion between the inner half part and the outer half partcan be reduced, and therefore, strength can be enhanced.

The present invention is not limited to the embodiment described above,and various additions, modifications, or deletions may be made withoutdeparting from the gist of the invention. For example, the air intakeduct of the present invention is also applicable to saddle-riding typevehicles other than motorcycles, and is also applicable to three-wheeledvehicles or four-wheeled vehicles. Therefore, these are construed asincluded within the scope of the present invention.

REFERENCE NUMERALS

-   -   24 . . . air intake port    -   24 a . . . outer side wall    -   30 . . . cylinder block    -   42 . . . supercharger    -   67 . . . connecting portion    -   69 . . . cleaner element    -   70, 70A . . . air intake duct    -   70 c . . . downstream portion    -   70 i . . . inner side wall of outermost side portion    -   A . . . incoming wind    -   E . . . combustion engine    -   I . . . intake air

1. An air intake duct of a saddle-riding type vehicle for supplyingincoming wind as an intake air to a supercharger for a combustionengine, comprising: an air intake port defined at a front end thereof;and a downstream portion defined at a rear end thereof, in which apassage area is gradually reduced from the air intake port toward thedownstream portion connected to the supercharger.
 2. The air intake ductof a saddle-riding type vehicle as claimed in claim 1, wherein the airintake duct extends from the air intake port disposed on one side of avehicle body, and passes laterally of one side of a cylinder block, tointroduce the intake air to the supercharger disposed rearwardly of thecylinder block.
 3. The air intake duct of a saddle-riding type vehicleas claimed in claim 2, wherein an outer side wall of the air intake portis positioned outward of an inner side wall of an outermost side portionof the air intake duct.
 4. The air intake duct of a saddle-riding typevehicle as claimed in claim 1, further comprising a connecting portionprovided in the downstream portion, the connecting portion having anincreased passage area.
 5. The air intake duct of a saddle-riding typevehicle as claimed in claim 4, further comprising: a cleaner element forcleaning the intake air disposed in the connecting portion; and anoutlet, of the connecting portion, connected to the supercharger, theoutlet being set so as to have a passage area that is less than an areaof the cleaner element.
 6. The air intake duct of a saddle-riding typevehicle as claimed in claim 1, further comprising an introductionmember, for introducing the intake air to a suction port of thesupercharger, provided inside the air intake duct.
 7. The air intakeduct of a saddle-riding type vehicle as claimed in claim 1, wherein: theair intake duct forms a curve portion and is connected to thesupercharger rearwardly of the curve portion; and the air intake ducthas a transverse cross-section which is shaped such that a dimension inan orthogonal direction that is orthogonal to a radial direction of thecurve portion is gradually reduced from the center of the curve portiontoward an outer side, in the radial direction, of the curve portion. 8.The air intake duct of a saddle-riding type vehicle as claimed in claim1, wherein an opening edge of the air intake port extends rearwardly soas to be tilted outward in a vehicle widthwise direction in a planarview.
 9. The air intake duct of a saddle-riding type vehicle as claimedin claim 1, wherein: the air intake duct forms a curve portion and isconnected to the supercharger rearwardly of the curve portion; the airintake duct has a transverse cross-section of a dimension-reducing shapewhich is shaped such that a dimension in an orthogonal direction that isorthogonal to a radial direction of the curve portion is graduallyreduced from the center of the curve portion toward an outer side, inthe radial direction, of the curve portion; and the dimension-reducingshape is formed in at least a region in which a curvature of the curveportion is increased.
 10. The air intake duct of a saddle-riding typevehicle as claimed in claim 1, wherein: the air intake duct extends fromthe air intake port disposed on one side of a vehicle body, and passeslaterally of one side of a cylinder block, to introduce the intake airto the supercharger disposed rearwardly of the cylinder block; the airintake duct forms a curve portion and is connected to the superchargerrearwardly of the curve portion; and the air intake duct has atransverse cross-section in which an outer side edge bulges in a radialdirection of the curve portion so as to form an arcuate shape.
 11. Theair intake duct of a saddle-riding type vehicle as claimed in claim 10,wherein the transverse cross-section of the curve portion of the airintake duct comprises a D-shaped cross-section.
 12. The air intake ductof a saddle-riding type vehicle as claimed in claim 1, wherein: the airintake duct extends from the air intake port disposed on one side of avehicle body, and passes laterally of one side of a cylinder block, tointroduce the intake air to the supercharger disposed rearwardly of thecylinder block; and the air intake duct has a transverse cross-sectionwhich is a rectangular shape that has a longitudinal axis extending in avertical direction.
 13. The air intake duct of a saddle-riding typevehicle as claimed in claim 1, wherein: the air intake duct forms aoutwardly curve portion and is connected to the supercharger rearwardlyof the curve portion; and the air intake port includes an outermost sideend in a widthwise direction of the vehicle, which outermost side end isdisposed outward of an outermost curve portion of the air intake duct inthe widthwise direction.